1. Field of the Invention
This invention relates to video compression methods and apparatus, in particular to video compression methods involving compensation for errors.
2. Description of the Prior Art
A number of digital video compression standards are known, one group, known as MPEG, having been devised by and named after the Motion Picture Experts Group of the International Standards Organisation (ISO). In one form, known as MPEG1, early versions of the draft standard made use of forward prediction between past and current frames of a video signal. In later forms of MPEG1 and another known as MPEG2, the standard additionally makes use of bidirectional prediction which is a combination of forward and backward prediction with the prediction made as an average of the forward and backward predictive frames.
Further background information on MPEG coding can be found, for example, in ISO/IEC 11172-2:1993(E) "Information Technology--Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s".
Tests on picture sequences, for example MPEG picture sequences, have shown that the errors introduced by the quantizer/unquantizer function did not show a uniform distribution. In particular, the distribution of quantization errors is uneven over both spatial and temporal bands. This mean that errors created by some frames are lower than other frames.